A facing or surface turning head of this type is known from U.S. Pat. No. 3,824,883. This patent discloses a facing head where adjacently disposed slides, each equipped with a tool, are guided rectilinearly in the base body so that on tool adjustment, they execute a rectilinear (opposite) movement. An advantage of this construction is that the mass balancing takes place in the same axially defined radial plane. A disadvantage is that centrifugal force is generated during operation on the slides and must be overcome for adjustment. It must be remembered that the centrifugal force changes proportionally with the radial distance of the slides from the tool spindle axis. When the center of gravity of the slides traverses the tool spindle axis, a force direction reversal occurs which frequently disadvantageously manifests itself in the turned surface of the workpiece to be machined. The consequences are not only correspondingly large dimensions of the actuator and other components of the facing head but also greater stress on the bearings, increased surface load of the sliding faces, greater lubrication consumption and an irregular turning picture on the workpiece to be machined due to changing types of stress and load.
The above problems resulting from the centrifugal forces are also common to facing heads where slides actively serve as force balancing means positioned axially behind each other (as for example shown in DE-OS No. 3,408,352), or where slides are arranged centrally within each other as is disclosed in German Patent Application No. P 39 10 677.
Of course, the difficulties due to the centrifugal forces are particularly serious with high-speed facing heads, or facing heads with large diameter; both of which are being increasingly employed to obtain high-cutting speeds.
The journal "tz fuer Metallbearbeitung", 76th edition (1982), No. 3, p. 24-26, discloses a facing head in which the two slides serving for the mass compensation are not linearly displaced on tool adjustment but turned about an axis fixed with respect to the base body. Once these rotary slides are balanced with respect to their axis of rotation, the center of gravity of the rotary slides retains its position during the rotation; as a result no turning moment is developed and the drive force remains constant. The centrifugal forces of the rotary slides thus have no influence on the drive force. However, a disadvantage is that the cutting angles on tool adjustment change considerably because the tool cutting edge moves on a circular path. These relatively large cutting angle changes give poor cutting conditions, unstable cutting edges due to smaller wedge angle, and different surface qualities. The rotatability of the slides further results in a difficult design due to complex kinematics, an irregular advance per revolution, and inexact feed in longitudinal turning operations.
The invention is based on the problem of providing a facing head having two slides movable oppositely for mass balancing in which the centrifugal forces generated by the slides do not have any disadvantageous influence on the drive force for adjusting the slides and nevertheless an excessive change of the cutting angles on tool adjustment is avoided.
This problem is solved by the invention as found in claim 1.
A parallel crank mechanism is of course a "four-bar linkage" having two cranks which are rotatable about two spaced apart fulcrums and at their free ends are connected together by a connecting link (coupler), the two cranks having the same length and the distance between the fulcrums of the cranks being equal to the length of the connecting link. On a rotary movement of the cranks, each point of the connecting link likewise executes a rotary movement, but the connecting link remains parallel to itself.
In the facing head constructed according to the invention, a "double-sided" parallel crank mechanism is provided, the two cranks of which are made double-armed and carry at their free ends two connecting links forming the slides. Since the two slides are connected together via the cranks, the centrifugal forces generated by the slides do not have any influence on the drive force for adjusting the slides. When a tool adjustment is made, the slides remain parallel to themselves. On tool adjustment, the tool cutting edges change only slightly from cutting edges provided on the slides. In addition, the facing head formed according to the invention is distinguished by constructional simplicity, high function reliability and low production expenditure.
The invention is particularly suitable for high-speed facing heads or facing heads of large diameter.
Advantageous further developments of the invention are set forth in the subsidiary claims.